The invention relates to the adjustment of a desired engine torque by appropriate computation of the actuating variables especially for adjusting the air supply and the fuel supply to the engine in an engine having gasoline direct injection.
An important operating mode of an engine having gasoline direct injection is the approximately unthrottled operation with high air excess. The air mass in the combustion chamber is then substantially constant and the excess-air factor (lambda) as an index for the composition of the air/fuel mixture is determined by the injected fuel mass. The air mass in the combustion chamber, in combination with lambda and the rpm n, determines the torque developed by the engine. At high air excess, the desired torque can be adjusted for the most part via a variation of the fuel quantity. The combustibility of the mixture with high air excess is achieved by a spatially non-homogeneous mixture distribution. This mode of operation is characterized as stratified operation. In contrast to stratified operation, the operation with a homogeneous mixture distribution is without air excess or only with a slight air excess. The invention relates to the determination of actuating quantities in dependence upon the requested torque during stratified operation.
External requirements on the intake manifold pressure during stratified operation affect the air charge. Such requirements result, for example, from the situation that the exhaust-gas return and the tank venting require a certain pressure drop. The request which brings about the lowest intake manifold pressure is realized by a minimum selection and intervention into the throttle flap position.
If one leaves the fuel quantity, which is to be injected for a desired torque, unchanged, then lambda changes. This has unwanted torque changes as a consequence.
The task of the invention is to avoid the unwanted torque changes.
Advantageously, the determination of the actuating quantity xe2x80x9cinjection timexe2x80x9d is supplemented by a determination of the actuating quantity of the air supply.
In this way, the further task is solved, namely, to obtain an appropriate adjustment of the fuel supply and air supply to the engine to realize a pregiven engine torque while considering a maximum permissible value for the air number lambda.
This further task is solved by a supplemental limiting of the air supply to maximum values. This limitation guarantees the reproducible adjustment of low torques over a variation of injection pulsewidths. Without this limitation, an unwanted adjustment to too lean a mixture can result which could present problems with respect to the combustibility of the mixture and/or the exhaust-gas emissions.